The present invention relates to a photopolymerizable composition which contains, as essential constituents,
(a) a polymeric binder, PA1 (b) a polymerizable compound having at least one terminal, ethylenically unsaturated double bond and a boiling point above 100.degree. C. at standard pressure, and PA1 (c) a tricyclic N-heterocyclic compound as a photoinitiator. PA1 R.sup.2 is a hydrogen atom, an alkyl group, an aryl group, an aralkyl group or an aralkenyl group, PA1 R.sup.3 and R.sup.4 are identical or different, and each denotes a hydrogen atom, a halogen atom, an alkyl group or an alkoxy group, PA1 n is 0 or 1, and PA1 m is 1 or 2. PA1 R.sup.1 denotes phenyl, chlorophenyl or tolyl, PA1 R.sup.2 denotes methyl, phenyl or styryl, and PA1 R.sup.3 and R.sup.4 denote hydrogen, chlorine or bromine PA1 inhibitors to prevent thermal polymerization of the monomers, PA1 hydrogen donors, PA1 substances regulating the sensitometric properties of layers of this type, PA1 dyes, PA1 colored and uncolored pigments, PA1 color precursors, PA1 indicators, and PA1 plasticizers, such as polyglycols or esters of p-hydroxyl-benzoic acid.
In German Patent No. 20 27 467 (corresponding to British Patent. No. 1,354,541), photopolymerizable compositions are disclosed that comprise the above-mentioned constituents, but with specific derivatives of acridine and phenazine being used as the photoinitiators. Some representatives of this class of compounds are distinguished by a high photosensitivity. These preferred compounds have the disadvantage, however, of providing poor resolution of fine lines or screen dots.
German Patent No. 20 39 861 (corresponding to U.S. Pat. No. 3,765,898) discloses photopolymerizable compositions which contain derivatives of quinoxaline and quinazoline as photoinitiators. In the preferred compositions, these photoinitiator compounds yield only relatively low photosensitives.
European Patent No. 11 786 (corresponding to U.S. Pat. No. 3,272,609) discloses similar compositions containing quinoline derivatives as initiators. But the suitability of these compounds is not optimal, both as to image reproduction and to photosensitivity.